A particle beam irradiation system can be roughly classified into a particle beam irradiation system (for example, refer to PTL 1) including a synchrotron as an accelerator, and a particle beam irradiation system (for example, refer to PTL 2) including a cyclotron as an accelerator.
A particle beam irradiation system including a synchrotron includes an ion source; a linear accelerator; a synchrotron; a beam transport; a rotating gantry; and an irradiation apparatus. The synchrotron includes an annular beam duct, and the beam duct is provided with multiple bending magnets, multiple quadrupole magnets, a radiofrequency acceleration cavity, an extraction radiofrequency electrode, and an extraction deflector. The ion source is connected to the linear accelerator, and the linear accelerator is connected to the synchrotron. A portion of the beam transport, which is connected to an extraction port of the synchrotron, is installed in the rotating gantry, and communicates with the irradiation apparatus installed in the rotating gantry.
Ions (for example, protons or carbon ions) extracted from the ion source are accelerated by the linear accelerator. An ion beam generated by the linear accelerator is injected into the annular beam duct of the synchrotron. The ion beam turning through the beam duct is accelerated to a predetermined energy in the radiofrequency acceleration cavity to which a radiofrequency voltage is applied. A radiofrequency voltage is applied from a radiofrequency electrode of the extraction radiofrequency electrode to the ion beam which has turned around and reached the predetermined energy, thereby extracting the ion beam to the beam transport via the extraction deflector. A tumor volume of a patient on a treatment bed is irradiated with the ion beam from the irradiation apparatus. The rotating gantry rotates the irradiation apparatus such that a beam path of the irradiation apparatus coincides with an irradiation direction of the ion beam toward the target volume.
In a case where the target volume is divided into multiple layers in an irradiation direction of an ion beam, and each layer is scanned with an ion beam, a layer to which an ion beam has to reach is specified by changing the energy of the ion beam. As described above, the energy of an ion beam is adjusted by controlling the pattern of a radiofrequency voltage applied to the radiofrequency acceleration cavity, an excitation pattern of the quadrupole magnets, and an excitation pattern of the bending magnets. The scanning of the inside of each layer with an ion beam is controlled by adjusting an excitation current of an operation magnet provided in the irradiation apparatus.
A particle beam irradiation system including a cyclotron includes an ion source; a cyclotron; a beam transport; a rotating gantry; and an irradiation apparatus. The cyclotron includes a vacuum chamber formed of a pair of facing iron cores having a circular section; a radiofrequency acceleration apparatus; and an extraction magnet. The beam transport communicates with an extraction portion of the cyclotron in which the extraction magnet is disposed. The beam transport, the rotating gantry, and the irradiation apparatus of the particle beam irradiation system including a cyclotron have substantially the same structures of those of the particle beam irradiation system including a synchrotron.
In the particle beam irradiation system including a cyclotron, ions (for example, protons or carbon ions) extracted from the ion source are injected to the center of a section of the iron cores of the cyclotron, and are accelerated by the radiofrequency acceleration apparatus. An accelerated ion beam turns in a spiral pattern from the center of the iron cores toward an inner surface of a return yoke, and is extracted to the beam transport by the extraction magnet provided in a peripheral portion of the iron cores. A tumor volume of a patient on a treatment bed is irradiated with the extracted ion beam from the irradiation apparatus via the beam transport.
As described above, in a case where the target volume is divided into multiple layers, and each layer is scanned with an ion beam using the particle beam irradiation system including a cyclotron, the energy of an ion beam extracted to the beam transport is adjusted by using a degrader provided in the beam transport. The degrader is formed of a single metal plate or a combination of multiple metal plates having different thicknesses. The degrader reduces the energy of an ion beam passing through the degrader, that is, adjusts the energy of an ion beam with which the target volume is irradiated. Since the energy of an ion beam accelerated by the cyclotron typically is constant, the energy of an ion beam is increased to the maximum energy required for cancer treatment by the cyclotron, the energy is dampened and adjusted to a predetermined energy when the ion beam penetrates through a metal plate provided in the degrader.
PTL 3 discloses a cyclotron that is used in this type of particle beam irradiation system and is capable of improving ion beam extraction efficiency. The cyclotron includes a pair of magnetic poles between which ion beam turning trajectories are formed, which includes multiple protrusions and multiple recessions which are alternately disposed in a circumferential direction, and by which hill regions are formed interposed between the protrusions and valley regions are formed interposed between the recessions along the turning trajectories; dee electrodes which are provided in the valley regions; and an acceleration cavity that is disposed in at least one valley region other than the valley regions in which the dee electrodes are provided, and on an outer circumferential side in a radial direction of the ion beam turning trajectories, and accelerates an ion beam. In the cyclotron in which the acceleration cavity is provided in addition to the dee electrodes so as to accelerate an ion beam, a turn separation is increased by an increase in the amount of energy increase per one turn of an ion beam, and ion beam extraction efficiency is improved.
PTL 4 discloses a charged particle beam irradiation method in which a tumor volume is divided into multiple layers from a body surface of a patient in an irradiation direction of an ion beam, and multiple irradiation points inside each layer are irradiated with ion beams by scanning the multiple irradiation points with fine ion beams. An ion beam is moved to an adjacent irradiation point inside a layer by controlling a scanning magnet provided in an irradiation apparatus. An ion beam is moved from a distal layer to a proximal layer by changing the energy of an ion beam. A Bragg peak (to be described later) of an ion beam reaches a distal position of a target volume by the extent of the increase in the energy of the ion beam. In a case where the patient is irradiated with an ion beam, a dose distribution illustrated in FIG. 3 of PTL 4 is obtained in a depth direction from the body surface of the patient, a dose reaches the maximum value at a Brigg peak, and the dose distribution is rapidly decreased at a depth at which the Bragg peak is exceed. Cancer treatment via ion beams uses properties in which a dose reaches the maximum value at a Bragg peak and is rapidly decreased at a depth at which the Bragg peak is exceeded.
In a particle beam irradiation system disclosed in PTL 5, a circular accelerator which extracts ion beams is attached to a rotating frame which rotates in a vertical position, and a beam transport chamber is provided to guide ion beams, which are extracted from the accelerator, to a treatment room. The beam transport chamber is connected to an extraction port of the accelerator. The beam transport chamber extends in a radial direction of the accelerator, is bent toward a horizontal direction, and reaches a position directly above the treatment room, and thereafter, the beam transport chamber is bent downward. A beam delivery system is attached to a tip end portion of the beam transport chamber. The treatment room is formed inside a radiation enclosure, and a patient to be irradiated with ion beams lies on a treatment bed installed inside the treatment room. A side wall of the radiation enclosure is disposed between the accelerator and the treatment room. A target volume of the patient on the treatment bed is irradiated with ion beams which are extracted from the circular accelerator and transported via the beam transport chamber and the beam delivery system. In order to change an irradiation direction of an ion beam, the direction of the beam delivery system is changed by rotating the accelerator via rotation of the rotating frame, and turning the beam transport chamber and the beam delivery system around a rotational center of the accelerator.